Review: Physiology and nutrition of late gestating and transition sows

Abstract

The physiology during late gestation and the transition period to lactation changes dramatically in the sow, especially during the latter period. Understanding the physiological processes and how they change dynamically as the sow approaches farrowing, nest building, giving birth to piglets, and producing colostrum is important because these processes greatly affect sow productivity. Glucose originating from assimilated starch accounts for the majority of dietary energy, and around farrowing, various organs and peripheral tissues compete for plasma glucose, which may become depleted. Indeed, physical activity increases shortly prior to farrowing, leading to glucose use by muscles. Approximately ½ to 1 d later, glucose is also needed for uterine contractions to expel the piglets and for the mammary gland to produce lactose and fat for colostrum. At farrowing, the sow appears to prioritize glucose to the mammary gland above the uterus, whereby insufficient dietary energy may compromise the farrowing process. At this time, energy metabolism in the uterus shifts dramatically from relying mainly on the oxidation of glucogenic energy substrates (primarily glucose) to ketogenic energy supplied from triglycerides. The rapid growth of mammary tissue occurs in the last third of gestation, and it accelerates as the sow approaches farrowing. In the last 1 to 2 wk prepartum, some fat may be produced in the mammary glands and stored to be secreted in either colostrum or transient milk. During the first 6 h after the onset of farrowing, the uptake of glucose and lactate by the mammary glands roughly doubles. Lactate is supplying approximately 15% of the glucogenic carbon taken up by the mammary glands and originates from the strong uterine contractions. Thereafter, the mammary uptake of glucose and lactate declines, which suggests that the amount of colostrum secreted starts to decrease at that time. Optimal nutrition of sows during late gestation and the transition period should focus on mammary development, farrowing performance, and colostrum production. The birth weight of piglets seems to be only slightly responsive to maternal nutrition in gilts; on the other hand, sows will counterbalance insufficient feed or nutrient intake by increasing mobilization of their body reserves. Ensuring sufficient energy to sows around farrowing is crucial and may be achieved via adequate feed supply, at least three daily meals, high dietary fiber content, and extra supplementation of energy.

The transition period is characterized by marked changes in physiology and nutritional needs within a short period of time. This review presents the current knowledge on important organs such as the uterus, mammary gland, and liver and how nutrition may improve the birth process and colostrum production of sows.

Introduction

The transition period attracts special interest because it has a huge impact on sow productivity. The transition period is not well defined but spans approximately the last 5 to 7 d prior to farrowing until 3 to 5 d after farrowing. During the transition period, the sow’s physiology undergoes substantial changes as does the physiology of the offspring. The impact of nutrition during the transition period on the farrowing process is becoming a hot research topic due to the increased rate of piglet mortality in modern hyperprolific sows, which also constitutes a major economic and welfare concern to the swine industry. Furthermore, sow milk yield is limiting the growth rate of suckling piglets (Wolter et al., 2002) and needs to be maximized via increased mammary development in late gestation. Consequently, nutritionists are exploring effective feeding strategies during the transition period to improve the survival and growth of the piglets. Complications during farrowing are the key driving force for increased stillbirth rate, and this was clearly demonstrated by Friendship et al. (1990) in a series of studies with over 800 cesarean sections, where only 1.9% of the piglets were delivered dead. The current review focuses on the drastic changes that occur in the sow during the transition period and on how these changes should be taken into consideration from a nutritional standpoint. A few organs/organ systems are of major interest to understand these physiological changes, namely the uterus (which also includes fetuses and placenta prior to farrowing), the mammary glands, and the liver. The physiological requirements of sows change very rapidly during the transition period because 1) mammary and fetal growth is accelerated; 2) the sow performs nest building; 3) the uterus expels piglets and placenta during the birth process and undergoes rapid regression after farrowing is completed; 4) the mammary glands produce and secrete colostrum and, subsequently, fat-rich transient milk; and 5) the liver physiology changes dramatically around farrowing. The day of farrowing is particularly demanding for the sow in terms of energy expenditures. Hence, glucose is a limited resource because it is utilized for nest-building activity prior to the onset of farrowing, for uterine contractions, and for the synthesis of colostral lactose and fat. This review will present the most current knowledge on this increasingly important aspect of nutrition of sows during the transition period.

Nutritional Impact on Mammary Development

The number of mammary cells present at farrowing has an impact on potential sow milk yield (Head and Williams, 1991) and can be altered by nutrition. However, it is only when mammary development is already taking place that nutrition can play a role. There are three stages in the pig’s life where there is rapid accretion of mammary tissue, namely from 90 d of age to puberty, from day 90 of gestation until farrowing, and during lactation (King et al., 1996; Sorensen et al., 2002). During the last third of gestation, mammary glands undergo major histological changes with adipose and stromal tissues being largely replaced by milk-synthesizing tissue (Kensinger et al., 1982; Ji et al., 2006). Hence, the feeding strategy used during late gestation is critical to enhance sow performance.

Early studies showed a negative impact of feeding too much energy as of day 75 of gestation (43.96 vs. 24.12 mega joules metabolizable energy [MJ ME]/d) on mammary development (Weldon et al., 1991). This effect was mostly related to body condition because when protein and energy intakes were manipulated during gestation to create obese (36 mm backfat) or leaner (24 mm backfat) gilts, even though mammary tissue weight close to farrowing was unaffected by body condition, there was a 3-fold decrease in mammary DNA concentration in overly fat gilts (Head and Williams, 1991), indicating that obese sows had reduced mammary cell division. When comparing body conditions that better reflect what is commonly seen in pig herds nowadays, Farmer et al. (2016b) demonstrated that providing different amounts of feed to gilts throughout gestation (1.30, 1.58, or 1.82 times maintenance requirements) to achieve backfat thicknesses of 12 to 15 mm (lean), 17 to 19 mm (medium), or 21 to 26 mm (fat) on day 109 of gestation altered their mammary development. Parenchymal tissue (containing milk-synthesizing cells) mass was significantly reduced in lean gilts, being 1,059, 1,370, and 1,444 g for lean, medium, and fat gilts, respectively. The importance of body condition was corroborated in a comparative study where relations between backfat thickness and mammary development were investigated in late-pregnant gilts (Farmer et al., 2017). Feeding level in gestation was determined to be more important than the body condition of gilts at mating for parenchymal tissue weight at the end of gestation (Farmer et al., 2016a), and it should, therefore, be adjusted for gilts to achieve an optimal range of backfat (17 to 26 mm) before farrowing. In contrast, recent Danish recommendations for modern sow genotypes have gone from 16–19 to 14–17 mm backfat at the entrance in the farrowing unit (Hojgaard and Bruun, 2021). Special care will, therefore, need to be taken to verify that proper mammary development and milk yield are achieved under such lean body conditions. The metabolic status of the late-pregnant gilt can also favorably affect mammary development. When circulating concentrations of the growth factor insulin-like growth factor-1 (IGF-1), which is an indicator of energy status, were increased via injections of porcine somatotropin from days 90 to 110 of gestation, mammary parenchymal mass was significantly augmented from 1,576 to 1,922 ± 124 g (Farmer and Langendijk, 2019). Certain feed ingredients could be used as a tool to increase IGF-1 concentrations in gilts, and nutrition companies are currently exploring that possibility.

Increasing protein intake (330 vs. 216 g crude protein/d; Weldon et al., 1991) or providing 4, 8, or 16 g/d of Lys (Kusina et al., 1999a) in late gestation did not affect mammary development of gilts. However, in that latter study, the 4 and 8 g/d treatments failed to support maximal milk yield in the subsequent lactation (Kusina et al., 1999b). When 20.6 g/d instead of 14.7 g/d of standard ileal digestible (SID) Lys was fed to sows as of day 90 of gestation, subsequent piglet weight gain was increased (Che et al., 2020). Considering that in the last 12 d of gestation, mammary growth accounts for 16.8% of SID Lys requirements (Feyera and Theil, 2017), this improved piglet weight gain could be due to enhanced mammary development. Krogh et al. (2017) showed that mammary glands had an uptake of 3 and 6 g/d of Lys 10 and 3 d before expected farrowing, respectively. This Lys uptake is most likely utilized for mammary growth because colostral proteins are taken up mainly as intact immunoglobulins and growth factors. Recent findings demonstrated that feeding 2.65 kg/d of a high-Lys diet (supplying a total of 26.0 g/d of SID Lys) compared with a conventional diet (18.6 g/d of SID Lys) increased total mammary parenchyma by 44% and significantly increased total parenchymal fat, protein, DNA, and RNA (Farmer et al., 2022). This high-Lys diet also contained more crude protein (21.4% vs. 15.4%) because it had more soybean meal. Such findings are most important because they indicate that current Lys requirements (NRC, 2012) are underestimated with regard to mammary development in late gestation and that a two-phase feeding strategy should be used to better meet the needs of preparturient sows. However, the greater concentrations of other amino acids in the high-Lys diet may have also come into play, and this needs to be further investigated.

There are feeding contraindications in late gestation to ensure optimal mammary development and lactogenesis. Prolactin is a hormone that is essential for mammogenesis as of day 90 of gestation (Farmer and Petitclerc, 2003), and it is also required for the initiation of lactation from day 110 of gestation until farrowing and for the maintenance of lactation postpartum (Farmer et al., 1998). Hence, any feed ingredient that decreases circulating concentrations of prolactin, such as ergots (Oresanya et al., 2003), is detrimental. Sows consuming ergotized barley in late gestation had almost no mammary development and showed agalactia (Nordskog and Clark, 1945). This was corroborated in another study when 1.5% sorghum ergot was fed to sows for the last 14 d prepartum, as it compromised mammary development and milk yield (Kopinski et al., 2007).

Interestingly, feeding in late pregnancy can affect the mammary development of the female offspring at puberty. Dietary supplementation with 10% flaxseed from day 63 of gestation until the end of lactation tended to increase the mammary parenchymal mass of the female offspring at puberty and increased parenchymal protein content (Farmer and Palin, 2008). It is evident from the literature that nutrition in late gestation needs to be considered in order to maximize mammary development and milk yield of sows.

Nutritional Impact on Fetal Growth

Fetal weight and accretion of nutrients in the fetal body increase exponentially during late gestation. It was shown that 35% of the increase in fetal weight occurs during the final 10 d of gestation (Wu et al., 1999; McPherson et al., 2004), suggesting that nutrient supply during this period needs to be adjusted to meet the exponential growth of the piglets. However, piglet and litter birth weight seems almost irresponsive to sow nutrition, indicating that the sow body may be used as a buffer of nutrients if the dietary supply is insufficient. Mallmann et al. (2019) reported that mean piglet birth weight increased 2.1% (from 1,300 to 1,327 g) if gilts were fed 2.3 kg/d when compared with 1.8 kg/d, and no further increase in birth weight was achieved using either 2.8 or 3.3 kg feed per day from day 90 of gestation until farrowing. Gonçalves et al. (2016b) reported a slight increase from 1.28 to 1.31 kg (2.3%) in birth weight of live-born piglets when sows were fed 3.8 kg/d from day 113 of gestation and until farrowing, while control sows received 2.7 kg/d. In a recent dose–response trial, neither piglet nor litter birth weights were affected significantly when gilts and sows were fed increasing levels going from 1.8 to 5.0 kg/d during the last week of gestation (Feyera et al., 2021b), but the mean birth weight increased numerically by 6% from the lowest feed intake to the highest feed intake (1.16 to 1.24 kg birth weight, respectively). A large-scale study reported that increasing the feed supply from 2.7 to 3.8 kg/d either at day 107 or at day 113 until farrowing could increase the mean piglet birth weight by 5.2% to 5.7% when compared with offspring being born from sows fed 2.7 kg/d until farrowing (Gourley et al., 2020a).

A review by Gonçalves et al. (2016a) showed that high feed intakes in late gestation could numerically increase piglet birth weight in litters from gilts but not from sows. The observation that piglet and litter birth weights were only marginally improved by increased feeding levels in gilts but not affected in sows may reflect the difference in the buffering capacity between gilts and sows when prioritizing fetal growth during late gestation, a period when nutrient supply is limiting. Even though piglet and litter birth weights of gilts and sows are generally irresponsive to the level of feeding in late gestation, it is highly recommended to feed late gestating gilts and sows optimally with adequate feed and proper feed composition to avoid excessive mobilization of body reserves, which will negatively affect subsequent reproductive performance. However, it is unlikely that substantial improvements in piglet birth weight may be achieved by altered feed strategies or feed composition, especially for the sows. The findings described above also suggest that fetal growth is prioritized above mammary growth with respect to amino acid utilization and that gilts need to prioritize their own body partially at the expense of fetal growth.

Nest Building

During the last 10 to 36 h before the onset of farrowing, the sow performs nest building (P. K. Theil; personal observations; Thodberg et al., 2002). Nest building is associated with a great increase in physical activities (posture changes, standing, biting pen fittings, and restless behavior; Andersen et al., 2014), and these, in turn, increase the total heat production. To the best of our knowledge, heat production has not been measured during nest building, but a change in posture from lying to standing is known to increase heat production by 12 to 15 kJ/min (Noblet et al., 1993; Theil, 2002), which corresponds roughly to a doubling in total heat production per minute (Figure 1). Using surgically modified sows to study the uptake of energy metabolites in the hind leg, we have found that glucose and lactate accounted for approximately 80% of the carbon uptake when sows were lying (Theil et al., unpublished data). When sows were standing, the blood flow to the hind leg increased substantially, and glucose was the only supplier of glucogenic energy (glucose + lactate). It accounted for more than 80% of the carbon uptake, whereas lactate uptake was negligible. Thus, glucose is an important resource for sows, and this is particularly true just prior to farrowing, when the increased physical activity due to nest building may compromise plasma glucose. A decrease in plasma glucose concentrations was indeed observed 1 h after the onset of farrowing in sows with farrowings that started late (>6 h) after ingestion of the last meal prepartum (Feyera et al., 2018) as compared with farrowing that started earlier. From a physiological and evolutionary standpoint, sows do not aim at prioritizing glucose for nest building, but this happens nonetheless because glucose is a limited resource used for nest building, which occurs immediately before the uterus needs glucose for uterine contractions and before the mammary glands utilize glucose for the synthesis of colostral lactose and fat. The importance of nutrition for nest building and especially the potential impact of high-fiber diets and straw supplementation on attenuating nest building activity needs to be explored from an energetic point of view.

Figure 1.

Impact of lying and standing activity on sow heat production (HE, heat energy). HE-rest is the heat production during resting (lying posture), and HE-St. Act. is the heat production while sows are standing. The heat production was measured in respiration chambers in 360 consecutive 4-min intervals (i.e., 24 h), and a photocell recorded whether sows were lying or standing (sitting could not be distinguished from standing in this experiment). Nest building is associated with great increases in physical activity and is associated with increased oxidation of glucose.

Uterine Physiology and Nutritional Impact on the Farrowing Process

Physiology of the uterus and uterine uptake of nutrients

In modern hyperprolific sows under continuous selection for large litters, the metabolic demand of the uterus is very high in order to nourish the numerous growing fetuses and give birth to as many viable live-born piglets as possible. Changes in the profile of reproductive hormones are most prevalent during the last week of gestation in connection with preparing the sow and the uterus for the upcoming farrowing. Shifts in the profile of reproductive hormones trigger the final stage of maturity for the fetal pig to activate its pituitary and adrenal glands to produce corticosteroids that will be transported to the placenta to activate the production of prostaglandins (King and Wathes, 1989). These prostaglandins will in turn activate the regression of corpora lutea to terminate pregnancy via the withdrawal of progesterone, thereby allowing reproductive hormones involved in the initiation of uterine contractions to initiate the farrowing process (First and Bosc, 1979; Anderson, 2000).

Sparse information is available with respect to the control of uterine contractions during farrowing (Vallet et al., 2010), but sustained uterine contractions are important for rapid delivery of the piglets (Lawrence et al., 1995). On average, the uterus contracts four to five times with an intensity of 9.4 mm Hg for a period of 12 s to give birth to a single piglet (Mota-Rojas et al., 2005a, 2005b, 2007; Olmos-Hernandez et al., 2008). Undoubtedly the contractions of uterine smooth muscles require energy, although the amount of energy specifically required by the uterus during farrowing remains unknown. According to Kelley et al. (1978), multiparous sows kept under thermoneutral conditions spend 18 KJ NE/min for the farrowing process. However, this study did not indicate whether the energy expenditure was the overall energy cost of the farrowing process (contraction of both abdominal and uterine muscles) or the energy cost explicitly required for contractions of uterine smooth muscles (Ganong, 2005; Vallet et al., 2010). Sow exhaustion due to depletion of readily available energy during farrowing was suggested to impair uterine contractions (van Kempen, 2007), thus delaying the farrowing process and increasing the stillbirth rate (Feyera et al., 2018). Therefore, a supply of the optimal amount of energy is important to speed up the farrowing process and reduce stillborns.

Using a multi-catheterized sow model implanted with catheters at the femoral artery and the main vein draining the right side of the uterine horn, we reported uterine extraction rates of the major energy metabolites in late gestating and farrowing sows (Feyera et al., 2018). There was a major net uptake of glucose, and minor uptakes of lactate, nonesterified fatty acids, butyrate, and glucose accounted for more than 90% of the net carbon uptake (Figure 2; Feyera et al., 2018) during the last month of gestation. Such findings indicate that the uterus strongly favors the oxidation of glucogenic energy and that glucose supplies the vast majority of carbon, and hence energy, to the uterus prior to the onset of farrowing. In support of these findings, Ford et al. (1984) observed a constant uptake of glucose by the gravid uterus of sows during the last 22 d of gestation, and Lindsay (1975) and Père and Etienne (2018) reported that in late gestation the sow uterus relies largely on carbohydrates for oxidative purposes, instead of other energy substrates. Interestingly, Feyera et al. (2018) found increased uterine extraction of nonesterified fatty acids. This suggests increased oxidation of ketogenic energy as the contribution of carbon from nonesterified fatty acids increased from 2% to 11% from 28 to 3 d prepartum, and acetate also started to contribute with 2% of the carbon uptake in the last week (Figure 2). These observations reflect that the uterus adapts physiologically to have a greater reliance on ketogenic energy as farrowing approaches, and this could be an important physiological adaptation. At farrowing, glucogenic energy becomes scarcely available, because the uterus competes with the mammary glands for glucose (Feyera et al., 2018, 2019), and glucose has already been utilized for nest building. The uterine extraction rate of acetate increased from −6.4% (the negative value means that acetate was released into the blood) to +5.1%, and the uterine extraction rate of butyrate increased from 8.4% to 18.6% from 28 to 3 d prepartum. Interestingly, both acetate and butyrate were released from the uterus during farrowing, which shows that short-chain fatty acids from fermented fibers do not directly supply fuel molecules for the hard-working uterus, not even in sows fed a high fiber.

Figure 2.

The uptake or release of moles of carbon by the uterus during late gestation, farrowing, and the post-farrowing period. The uptake or release of moles of carbon was expressed as a percent of total uptake or release of moles of carbon, respectively. Farrowing refers to the time from the onset until the end of farrowing, whereas post-farrowing refers to the period from the end of farrowing until 24 h after the onset of farrowing.

It was surprising to note that triglycerides and glucose appeared to be the only two energy substrates for oxidation by the uterus during farrowing (Feyera et al., 2018) and that carbon originating from triglycerides contributed as much as 61% of the total carbon taken up by the uterus during farrowing. Normally, when sows become catabolic, nonesterified fatty acids are released from fat depots and used to supply energy to peripheral tissues, but during farrowing, triglycerides merely than nonesterified fatty acids supplied the energy. The most likely explanation is that the uterus is in a really short supply of glucogenic energy and takes up trigycerides, because the glycerol part of triglycerides may be used as glucogenic energy. The fact that the uterus relies more on ketogenic than glucogenic energy on the day of farrowing strongly indicates that the sow lacks glucose as it is used for nest building and prioritized for colostrum production, whereby the supply to the uterus is strongly compromised. It is possible that feeding high dietary fiber to late gestating sows could be beneficial for the uterus during farrowing because the uterus has a net uptake of acetate and butyrate in the last week of gestation likely allowing for ketogenic oxidation. Another beneficial effect of high fiber is that short-chain fatty acids are absorbed from the gastrointestinal tract for an extended period after ingestion of the last meal, and this reduces the diurnal variation in blood glucose (Serena et al., 2009). This stabilizing effect of fiber on plasma glucose most likely involves the metabolism of propionate into lactate or glucose, which are both glucogenic substrates. When there is an extended time span between the last meal and the onset of farrowing, the liver glycogen depot could most likely buffer the plasma glucose via net absorbed propionate. However, depletion of liver glycogen depots could lead to a dramatic drop in plasma glucose, as low as 2 mmol/L in the worst-case scenario (Feyera et al., 2018), and this may fatally impair contractions of the uterine smooth muscles during farrowing and increase piglet mortality. The infusion of glucose was recently demonstrated as an effective strategy to assist farrowings in sows that were fed a single daily meal during the last week of prepartum (Nielsen et al., 2021). Carbons released from the uterus to the blood as CO₂ (55% to 78% of carbon release) or as triglycerides (22% to 43% of carbon release) represented almost all carbons released during late gestation. On the day of farrowing, carbon released as CO₂ represented the majority of released carbon from the uterus (86%), while lactate, acetate, and nonesterified fatty acids only had small contributions (8.5%, 3.0%, and 1.7%) of carbon release, respectively.

In general, the arteriovenous differences and extraction rates of most energy metabolites in the uterus of sows during late gestation and farrowing appear very low (below 10%). However, this does not necessarily imply that the net uptake of energy metabolites by the uterus is also low, as it merely indicates that the blood flow during these periods is really high, most likely because O₂ is insufficiently supplied. Furthermore, extraction rates show a qualitative, and not quantitative, relationship with the metabolic demands of the uterus. According to Ferrell and Ford (1980) and Hard and Anderson (1982), blood flow is the primary determinant of nutrient availability to the uterus and is more informative than arteriovenous differences. Measuring total blood flow and quantifying the net nutrient uptakes by the uterus in sows are extremely challenging due to the complex nature of the uterine vascular system. Oxenreider et al. (1965) described that the whole length of the swine uterus is drained by three different veins, namely, according to their descending size, the utero-ovarian vein, uterine vein, and uterine branch of the urogenital vein. These last authors stated that the utero-ovarian vein drains the majority of the uterine horn, the uterine vein drains one-half to two-thirds of the uterine horn, and the uterine branch of the urogenital vein drains the main body of the uterus. Therefore, quantitative measurement of blood flow using downstream dilution of the uterine veins and net nutrient uptake in the uterus of pregnant sows requires the placement of six different venous catheters in a single sow and another six catheters for infusion of a blood flow marker. Understandably, it would be impossible to achieve a steady state while infusing the blood flow marker at several locations. Consequently, few of the blood flow measurements reported in the gravid uterus of sows involved the implantation of electromagnetic blood flow probes around the middle artery on one of the uterine horns (Ford et al., 1984; Reynolds et al., 1985; Père and Etienne, 2000, 2018). Thus, it could be logical to speculate that such blood flow measures may not necessarily reflect total blood flow to the whole uterine horn.

Père and Etienne (2000) reported a linear increase in blood flow to the gravid uterus with advancing stage of gestation, from 72 L/h on day 44 to 156 L/h on day 111 of gestation. These authors further indicated that uterine blood flow is more closely related to litter weight than to litter size and reported 0.42 L/min per kg of fetal weight but observed a decreased uterine blood flow per fetus with increasing litter size. This latter observation emphasizes the physiological challenge that the gravid uterus is facing in modern hyperprolific sows. Taking into account the uterine capacity, the decrease in uterine blood flow per fetus with increasing litter size partly justifies the declining birth weight with increasing litter size in modern sows. In a later study, Père and Etienne (2018) noted an increased uterine blood flow going from 120 L/h on day 79 to 159 L/h on day 106 of gestation. In contrast, older studies reported a constant blood flow (90 L/h) in the gravid uterus from day 70 of gestation until farrowing (Ford et al., 1984; Reynolds et al., 1985). Surprisingly, Ford et al. (1984) even observed a dramatic decline in uterine blood flow on the day of farrowing when compared with late gestation. The fact that fetuses are growing exponentially during late gestation (McPherson et al., 2004), combined with the lower extraction rates or arteriovenous differences of nutrients across the uterus during late gestation and farrowing (Feyera et al., 2018), and the high energy demand of farrowing (Ganong, 2005; Vallet et al., 2010), suggests that it is not likely that uterine blood flow remains constant during late gestation and farrowing.

It is important to consider that any aid to assist the farrowing process should not disrupt the normal physiology of uterine contractions. Injections of oxytocin and carbetocin after the birth of the first piglet are often used in commercial farms as farrowing aids to accelerate the farrowing process. However, several studies demonstrated an increased rate of stillbirths and the birth of less vigorous piglets in sows receiving oxytocin compared with control sows, and this is regardless of a significant reduction in farrowing duration (Mota-Rojas et al., 2005a; Boonraungrod et al., 2018; Jiarpinitnun et al., 2019). These studies indicate that oxytocin used to stimulate the farrowing process may interfere with the normal physiological rhythm of the myometrial muscle contractions. Exogenous oxytocin may be associated with uterine hyper-stimulation leading to a subsequent decrease in placental perfusion that compromises fetal survival, leads to dysfunctional labor and, in the worst-case scenario, to uterine rupture. Mota-Rojas et al. (2005a) observed a greater frequency (13-fold), increased intensity (2-fold), and increased duration of uterine contractions in sows receiving a single dose of oxytocin during farrowing when compared with sows giving birth naturally. Olmos-Hernandez et al. (2008) illustrated that a longer duration of uterine contractions that are less intense is distressful and has an adverse effect on the survival of the piglets due to an increased risk of umbilical rupturing and a greater occurrence of stillbirths. Uterine contractions during farrowing apparently restrict uterine blood flow and gaseous exchange in the utero-placenta; thus, a longer duration of uterine contractions without successful delivery of the piglet could increase fetal hypoxia, which is a risk factor for stillbirths and less vigorous piglets at birth. On the other hand, administration of carbetocin was reported to reduce sow colostrum yield and colostrum intake by the piglets (Boonraungrod et al., 2018; Jiarpinitnun et al., 2019). Even though the administration of oxytocin and carbetocin at the onset of farrowing accelerates the farrowing process, the use of these products is not generally recommended because of compromised survival of the neonatal piglets. In summary, the energy metabolism in the uterus relies normally on glucogenic energy, but during farrowing, an unexpected huge shift to oxidation of ketogenic energy occurs, which most likely is due to glucose being prioritized for colostrum production in the mammary gland.

Nutritional impact on the farrowing process

The metabolic demands of sows during the transition period are not well known because feeding during this period has received little attention in the past, unlike feeding during gestation and lactation. Sows are undergoing marked physiological and metabolic changes around farrowing, and nutrient requirements are also altered dramatically during this period (Feyera and Theil, 2017). The energy status of sows, as well as the energy available from the gastrointestinal tract and glucose availability, seems to be very important factors for a successful farrowing. Nutritional strategies to achieve a proper energy supply at farrowing are 1) adequate feed supply, 2) adequate number of daily meals, 3) use of high-fiber diets, and 4) extra supply of glucose/energy between meals shortly before farrowing. The efficiency of feeding strategies during the transition period should be evaluated according to farrowing dynamics such as farrowing duration, birth intervals, and farrowing assistance in hyperprolific sows. The rate of stillbirth is of major importance for farmers but it has such a low occurrence that it is difficult to use and see significant treatment effects. Another important aspect is avoidance of constipation, for which the addition of fiber to the diet is beneficial. It has been recommended to reduce feed supply during the last few days prior to farrowing to diminish the risk of constipation during farrowing (Tabeling et al., 2003); however, other studies showed that a low feed supply during the transition period is a risk factor to develop constipation at farrowing (Lee and Close, 1987; Pearodwong et al., 2016). Recent findings emphasize the importance of maintaining an optimal feed supply during the transition period (Feyera et al., 2021b), with possible improvement via adequate (high) fiber (Oliviero et al., 2009; Feyera et al., 2017) to increase piglet survival through faster farrowing. On the other hand, care should be taken to ensure that any dietary intervention used during the transition period to improve the farrowing process does not negatively affect colostrum production.

Using a multi-catheterized sow model and retrospective data analysis, we have demonstrated the importance of sow energy status, evaluated as the time from the last meal until the onset of farrowing and plasma glucose concentrations, during the transition period on the farrowing kinetics and piglet survival under Danish conditions (Feyera et al., 2018). Findings from this study revealed that a longer farrowing duration increases the rate of stillbirth. It was evident that glucose homeostasis in sows 1 h after the onset of farrowing was challenged, and sows appeared to be depleted of energy during nest building if too many hours had elapsed between ingestion of the last meal and onset of farrowing. Feyera et al. (2018) also demonstrated that farrowing duration was rather short (<4 h) if sows started to farrow within the first 3 h after consuming their last meal. Furthermore, these sows with short farrowings had minimal need for farrowing assistance and a low stillbirth rate (<5%). On the other hand, if sows started to farrow more than 6 h after ingestion of their last meal, the duration of farrowing was substantially prolonged with concomitant increases in farrowing assistance and stillbirth rate (10- and 2-fold, respectively). The clear message was that sows should start to farrow before the energy status becomes critically low, and the nutrition of modern sows should be improved to alleviate farrowing complications. Undoubtedly, correct feed supply and three daily meals, preferably supplied at regular intervals, will assist the sows through the demanding process of farrowing.

Adequate feed intake is not only the simplest but also likely the most efficient way of ensuring adequate energy supply for sows. A dose-response study recently showed shorter farrowing duration (P = 0.03), minimal farrowing assistance (P = 0.02), and lower stillbirth rate (P = 0.64) when sows were fed 3.7 to 4.1 kg/d during the last week of gestation (Feyera et al., 2021b). This study clearly showed that there is no need to reduce the feed supply prior to the onset of farrowing and that a constant feed supply between 3.5 and 4.0 kg/d in the last days of gestation appears optimal. Accordingly, Che et al. (2019) observed reduced farrowing duration in sows fed 3.2 kg/d from day 90 of gestation until farrowing when compared with sows fed 3.0 kg/d. The study by Feyera et al. (2021b) indicated that farrowing kinetics were compromised both at very low and very high feed supplies (1.8 and 5.0 kg/d). While energy depletion may extend farrowing duration in the case of insufficient feed supply, an excess of feed may also cause problems because undigested fibers in the hindgut may physically block the birth canal due to extensive gut fill. In agreement with this, others reported a negative effect of excess feed supply on farrowing duration and rate of stillbirth (Miller et al., 2004; Liu et al., 2020).

Increasing the number of daily meals is a way to minimize the number of hours from consumption of the last meal until farrowing starts and also ensures that sows are not being depleted of energy during nest building. However, increasing the number of daily meals may not be that straightforward as it depends on the feeding systems, and more daily meals also mean that less amount of feed is supplied per meal, which in some feeding systems may be an obstacle. Another problem is that barn people either have to or want to be physically present during feedings and many farms that feed three daily meals do so during the working hours, which is less efficient for the sow than if three daily meals are supplied at 8 h intervals. Meal frequency has been studied in a large-scale experiment with a total of 727 sows being fed either ad libitum or restrictively, with 2.7 kg/d fed either in a single meal or in four daily meals at 6 h intervals (Gourley et al., 2020b). Farrowing assistance was greatest for sows fed ad libitum, intermediate for sows fed 1 daily meal, and lowest for sows fed 4 meals/d, with 19.6%, 16.1%, and 13.7% of piglets needing assistance, respectively (P < 0.001). Farrowing duration or the incidence of stillbirths was not significantly affected by feeding treatment, although small numerical changes did support that the farrowing process was improved in feed-restricted sows fed four meals.

High fiber using a fiber-rich supplement based on sugar beet pulp and soy hulls in the last 2 wk of gestation (and supplying 19% fiber in the final ration in the last week) has been shown to effectively reduce the rate of stillbirths in a commercial farm setting, even though the incidence of stillbirths was already below the national average of 9.8% in Denmark (Feyera et al., 2017). The modes of action most likely involved less constipation and a more constant supply of energy from the gastrointestinal tract many hours after the last meal was consumed due to the uptake of short-chain fatty acids. As demonstrated by Oliviero et al. (2009), sows fed a high-fiber diet containing 7% crude fiber mainly from oat, sugar beet pulp, and wheat bran in late gestation are less likely to be constipated prior to farrowing because of increased intestinal activity and high water-binding capacity of soluble fiber. Thus, physical blockage of the birth canal due to hard feces appears not to be a challenge for farrowing sows fed high fiber in late gestation. Dietary supplementation with inulin over three reproductive cycles (Li et al., 2021) or a single reproductive cycle (Wang et al., 2016) shortened the farrowing duration and birth intervals. Concomitantly the number of live-born piglets increased. A lower stillbirth rate was demonstrated by Deng et al. (2021) when sows were fed 15% wheat aleurone from mating until farrowing. Recently, Liu et al. (2022) reported both shorter farrowing duration and shorter birth intervals in sows fed 33.5% dietary fiber instead of 17% dietary fiber from day 90 of gestation until farrowing. These last authors further demonstrated a positive correlation between plasma concentrations of propionate and farrowing duration, as well as plasma concentrations of short-chain fatty acids and glucose. These positive correlations substantiate the role of fiber in ensuring an adequate energy status of sows during farrowing. Interestingly, Liu et al. (2022) showed greater plasma concentrations of betaine in sows fed high fiber when compared with sows fed low fiber. A study with growing pigs showed that dietary betaine reduced the heat production and the energy requirement for maintenance and, consequently, increased energy retention (Schrama et al., 2003). This may also explain the benefits of feeding high fiber to sows approaching farrowing.

A study by Papatsiros et al. (2021) where 500 g Arbocel (a commercial product containing 65% fiber) was fed from day 104 of gestation until day 7 of lactation led to a shorter farrowing duration and less stillbirths as well as less crushed piglets in the first week of lactation. Greater plasma concentrations of insulin at farrowing were reported in sows supplemented with Arbocel. Because concentrations of insulin in the plasma are known to increase with increasing plasma glucose concentrations, these findings, therefore, indicate that sows fed high fiber during the transition period could have more stable plasma glucose. In agreement, de Leeuw et al. (2004) observed more stable plasma glucose levels both shortly and several hours after feeding in sows fed highly fermentable fiber compared with sows fed a standard diet. In both treatments, sows were fed every 12 h and plasma glucose concentrations were measured every hour. These authors reported that plasma glucose concentrations did not drop beyond the basal levels in sows fed high fiber, whereas they dropped to basal concentrations at 3 h and below basal levels at 7 h after feeding in sows fed the standard diet.

Many studies looked at the role of high fiber on the farrowing process, but the optimum level of fiber inclusion for gestating sow remains to be investigated. Systematic evaluations of different fiber sources in the farrowing process are also lacking. In a recent study by Feyera et al. (2021a), fibers originating from palm kernel expellers or soy hulls increased farrowing assistance (17.9% and 19.5%, respectively) when compared with sugar beet pulp or a mixed fiber source based on sugar beet pulp, oat hulls, and fibers from wood (3.2% and 3.4%, respectively). Yet, no significant responses were seen on farrowing duration or stillbirths. The current state of knowledge indicates that the inclusion of high fiber in sow transition diets benefits the farrowing process, yet the ideal type and level of fibers to be used still need to be defined. According to a study carried out by Tydlitát et al. (2008), the farrowing duration increased from 4.5 to 8.6 h when the dietary protein increased from 13% to 21%, and this may be ascribed to poor utilization of protein when oxidized (Pedersen et al., 2019b). However, it may also be explained by the decreasing dietary fiber, which was confounded with increasing levels of dietary protein in that study.

Supplementation of energy prior to and during farrowing may improve the farrowing process in sows. The infusion of glucose to sows during nest building and farrowing significantly reduced farrowing assistance from 21% to 9% and stillbirths from 16.1% to 7.4% of total born piglets (Nielsen et al., 2021), but this approach is mainly interesting from a scientific point of view. In practice, providing a source of sugar to sows in labor may be beneficial. We have tried to supply extra energy from either sugar or glycerol between the three daily meals so that sows received energy every fourth hour, alternating between solid feed and water with supplemental energy (Feyera et al., 2021a). However, the extra energy supply did not improve farrowing duration or frequency of farrowing assistance or stillbirths. This absence of effect was most likely because the energy was diluted in water that was in the feed trough, thereby not ensuring that sows did ingest extra energy between the solid meals. Providing a readily available energy source on the expected date of farrowing tended to reduce farrowing assistance, farrowing duration, and stillbirths in sows that were induced to farrow (Oliveira et al., 2020). In summary, sufficient energy is highly important for the sow during farrowing, and supply of adequate amount of feed, sufficient number of daily meals, slow release of energy from fiber, and addition of extra energy at farrowing are all tools that seem to have potential to improve the farrowing.

Mammary Uptake of Nutrients

The mammary glands are of paramount importance for the sow during lactation due to the demanding milk production, but already in late gestation, the plasma flow and nutrient uptake to the mammary glands increase greatly. Krogh et al. (2017) showed that the plasma flow to the mammary glands increased from approximately 130 L/h on day −10 to 175 L/h on day −3 and then to 290 L/h on day 3 of lactation. Feyera et al. (2019) reported that the mammary plasma flow increased from 196 L/h 1 h after the onset of farrowing, peaked at 259 L/h 6 h later, and declined to reach a nadir of 180 L/h 18 h after the onset of farrowing (Figure 3A). The uptake of glucose by the mammary glands decreased in the last 28 d of gestation (Figure 3B) to then increase steadily from 42 to 96 mmol/h during the first 6 h after farrowing onset. This increase in glucose uptake reflects the greater synthesis of colostral lactose and fat in response to the enhanced suckling intensity and to colostrum being removed by newborn piglets. From 6 h until 24 h after the onset of farrowing, mammary glucose uptake decreased to 63 mmol/h, suggesting that mammary synthesis of lactose, and hence the volume of colostrum, also most likely decreased. The net mammary uptake of O₂ was fairly constant in late gestation (400 to 500 mmol/h) and was slightly elevated during the first 6 h after the onset of farrowing (500 to 600 mmol/h), indicating an increased metabolic activity. Interestingly, the mammary CO₂ production peaked in late gestation, at around 600 mmol/h between days −14 and −10, and the respiratory quotient (RQ) peaked at 1.4 at day −10 (Figure 3C). Normally, an RQ of 1.0 reflects carbohydrates being oxidized and lower RQ values indicate oxidation of fat or protein (RQ values of 0.7 and 0.82, respectively), whereas RQ values above 1.0 show that the mammary glands synthesize fat. Krogh et al. (2017) reported an RQ value of 1.2 in the mammary glands at day −10 in addition to a positive carbon balance, which together supports that mammary glands synthesize de novo fat. In late gestation and during the farrowing process, the mammary glands take up lactate, and this uptake peaks 6 h after the onset of farrowing (Figure 3D), thereby coinciding roughly with the end of farrowing. At this time, lactate uptake by the mammary glands begins to markedly drop and strongly indicates that the lactate originates from smooth muscles because of the intense uterine contractions around farrowing, and in support, Feyera et al. (2018) reported the release of lactate from the uterus at the day of farrowing. In late gestation, the uptake of amino acids (AA) by mammary glands was shown to be minimal, whereas it was much greater during lactation and reflected the amount of AA being secreted through milk proteins (Krogh et al., 2017). It is, however, questionable whether measurable amounts of AA are taken up by the mammary glands during colostrogenesis because most colostral proteins are likely not synthesized by the mammary glands but merely taken up as large intact molecules (immunoglobulins and growth factors). In the study by Feyera et al. (2019) uptake of neither immunoglobulins nor the IGF-I growth factor could be detected, but plasma concentrations of immunoglobulins decreased substantially and plasma concentrations of IGF-I increased substantially from 3 d prepartum to 1 h after the onset of farrowing, which support that these molecules are not synthesized in the mammary gland. In summary, the mammary gland produces colostrum on the day of farrowing and contributes greatly to the competition among organs with respect to glucose utilization. The sow mammary gland produces some fat prepartum, and this suggests that fiber-rich diets to late gestating sows may be a way to enhance colostral fat content and concomitantly increase the colostrum production and reduce the competition for glucose between the uterus and the mammary gland.

Figure 3.

Changes in mammary plasma flow (L/h; A), net mammary glucose uptake (mmol/h; B), mammary respiratory quotient (CO₂ production divided by O₂ consumption; C), and net mammary lactate uptake (mmol/h; D) in late gestation and at the day of farrowing. Note that prior to farrowing, time refers to days prepartum (day −28 to −3), while time postpartum refers to hours after the onset of farrowing.

Colostrum Production and Colostrum Intake

Colostrum is regarded as the “elixir of life” for newborn piglets and without sufficient colostrum intake, the risk of dying increases dramatically (Quesnel et al., 2012; Theil et al., 2014b), and a high colostrum intake has been shown to stimulate the growth of piglet until weaning and even until slaughter (Krogh et al., 2016b). Modern sows have been bred for hyper-prolificacy during the past 30 yr, and the litter size has increased by approximately 50% (DPRC, 2020). In these modern hyperprolific sows, where the number of live-born piglets exceeds the number of mammary glands, udder capacity for colostrum production is clearly the limiting factor (Figure 4; Krogh, 2017). During the past 13 yr, sow colostrum yield has increased from 5.3 to 6.4 kg (by approximately 21%) but this is insufficient to cope with the rapid increase in litter size and their demand for colostrum. Consequently, colostrum intake by piglets has decreased by 28%, from 443 g in 2008 to 319 g in 2020 (Theil et al., 2014a, 2022), and approaches the critical minimum amount of 250 g needed per piglet to ensure survival (Quesnel et al., 2012). Proper nutrition of transition sows is, therefore, highly important to make sure that sows meet the high demand for colostrum, leading to an increased survival rate of newborn piglets.

Figure 4.

Colostrum yield (A) and colostrum intake of piglets (B) in litters from hyperprolific sows in relation to the number of live-born piglets (Krogh, 2017).

For many years it was believed that colostrum was produced entirely before the first piglet was born so that colostrum was ready for consumption when farrowing started (Hartmann et al., 1997; Theil et al., 2014b). Based on the visual evaluation, the mammary glands grow rapidly during the last 5 to 7 d prepartum, which supported the hypothesis that colostrum was indeed produced prior to farrowing. A transcriptomic study of mammary biopsies taken every fourth day in late gestation also supported that hypothesis (Palombo et al., 2018). The study revealed that very few genes were differentially expressed between days −14 and −6 relative to expected farrowing, whereas approximately 800 genes were differentially expressed when comparing days −14 and −2, and more than 3,000 genes were differentially expressed when comparing days −14 and +1 (i.e., when the colostral period ends). The most upregulated genes included casein, α-lactalbumin, and Acyl-CoA synthetase long-chain family member 6, which are involved in the synthesis of colostral proteins, lactose, and fat, respectively, and whey acidic protein and butyrophilin, which are involved in regulating proliferation of mammary epithelial cells and secretion of milk lipids, respectively. However, a recent study with multi-catheterized sows, which allows studies on mammary uptake of nutrients, revealed that colostral fat and lactose are mainly produced after the onset of farrowing and the following 24 h (Feyera et al., 2019). In contrast, colostral immunoglobulins are produced and transferred to the mammary glands prior to farrowing. As a consequence, the sow plasma concentrations of immunoglobulin G (IgG) are considerably reduced at farrowing (Feyera et al., 2019) and need to be restored shortly thereafter. The colostral period is defined as the first 24 h after the first piglet is born, but it actually ends between 24 and 34 h following the onset of farrowing, at a time when copious milk production starts (Hartmann et al., 1997; Vadmand et al., 2015). The amount of colostrum secreted is greatest during the first 12 h after the onset of farrowing, and less colostrum is being secreted from 12 to 24 h (Krogh et al., 2015). The composition of the colostrum also changes dramatically with time (Hurley, 2015). The early colostrum is rich in IgG and growth factors (and hence protein) and is generally regarded as high-quality colostrum, whereas late colostrum (secreted 24 h or more after farrowing onset) is regarded as low-quality colostrum because the concentrations of IgG and growth factors have declined substantially (Hurley, 2015; Feyera et al., 2019). The macrochemical composition of early colostrum differs substantially from that of late colostrum and transient (and mature) milk. Early colostrum contains 24.4% to 29.0% dry matter, 3.9% to 6.9% fat, 14.2% to 20.4% protein, and 2.7% to 3.7% lactose, whereas late colostrum contains less dry matter and protein, and more fat and lactose (17.5%to 23.0% dry matter, 5.7% to 8.0% fat, 6.6% to 10.4% protein, and 4.2% to 4.6% lactose; Table 1). Late in the colostral period, the amount of colostrum being secreted is very low, and the colostral period ends abruptly when the sow starts to produce copious amounts of milk, which may be referred to as onset of lactation, stage II (Hartmann et al., 1997), and this happen between 22.5 and 39.4 h after onset of farrowing (Vadmand et al., 2015), although most agree for simplicity that the colostrum period comprises the first 24 h after the onset of farrowing. On days 2 through 4, sows produce transient milk with more fat and lactose than colostrum (17.9% to 20.6% dry matter, 7.4% to 9.4% fat, 4.8% to 6.1% protein, and 4.6% to 5.5% lactose). High energy supply and use of high-fiber diets for sows in late gestation and during the transition are important to enhance colostrum production of sows, but various fiber sources may act differently, and unraveling the mode of actions of the dietary fibers would be a way forward to enhance sow productivity because it is very labor demanding to study colostrum production.

Table 1.

Composition of early (0 to 1 h) and late (24 h) colostrum and transient milk (day 3)

Dry matter, %	Fat, %	Protein, %	Lactose, %	Dietary treatments	Study
Early colostrum
26.4 to 28.3	5.2 to 6.3	16.8 to 17.3	3.1 to 3.3	+/− CLA1	Krogh et al. (2012)
25.6 to 29.0	5.7 to 6.9	14.2 to 15.8	2.7 to 3.2	Fat level & source	Theil et al. (2014a)
26.2 to 27.5	4.7 to 5.3	17.0 to 18.0	3.5 to 3.6	Fiber & fat	Krogh et al. (2015)
24.9 to 27.0	4.9 to 5.4	15.8 to 16.7	3.6 to 3.7	+/− arginine	Krogh et al. (2016a)
27.0 to 28.7	4.3 to 5.2	19.5 to 20.4	2.7 to 3.0	Protein & energy	Che et al. (2019)
25.5	4.7	15.9	3.5	Fiber & mineral	Feyera et al. (2019)
24.4 to 27.9	3.9 to 5.4	15.7 to 17.7	3.5 to 3.7	Protein & energy	Krogh et al. (2020)
26.9 to 28.5	4.6 to 5.4	17.9 to 18.3	3.2 to 3.3	Protein level	Pedersen et al. (2020)
27.2	6.4	16.5	3.3	Feeding level	Feyera et al. (2021b)
26.7	4.8	17.2	3.3	Fiber sources	Feyera et al. (2021a)
25.9	4.0	17.3	3.3	Glucose infusion	Nielsen et al. (2021)
Late colostrum
19.0 to 23.0	5.7 to 7.5	7.3 to 10.0	4.2 to 4.6	Fiber & fat	Krogh et al. (2015)
19.3 to 20.9	7.2 to 7.5	7.8 to 8.8	4.3 to 4.4	+/− arginine	Krogh et al. (2016a)
20.6	7.5	9.7	4.2	Fiber & mineral	Feyera et al. (2019)
17.5 to 19.4	6.1 to 7.5	6.6 to 8.2	4.3 to 4.4	Protein & energy	Krogh et al. (2020)
19.0 to 20.4	6.6 to 7.4	7.2 to 8.2	4.3 to 4.4	Protein level	Pedersen et al. (2020)
20.0	8.0	7.0	4.4	Feeding level	Feyera et al. (2021a)
19.5	6.9	7.6	4.4	Fiber sources	Feyera et al. (2021b)
18.6	6.1	7.5	4.3	Glucose infusion	Nielsen et al. (2021)
Transient milk
19.2	8.4	5.6	4.8	Glucose infusion	Nielsen et al. (2021)
17.9 to 19.2	7.4 to 8.7	5.1 to 5.8	5.0 to 5.5	Protein	Strathe et al. (2020)
19.1	8.1	5.8	4.9	Protein	Pedersen et al. (2020)
18.2 to 18.7	7.8 to 8.5	4.8 to 5.5	4.6 to 4.8	Protein & energy	Krogh et al. (2020)
18.7 to 19.6	7.7 to 9.0	5.2 to 5.7	4.8 to 4.9	Protein	Pedersen et al. (2019a)
19.1	8.1	5.3	5.0	Protein	Hojgaard et al. (2019a)
18.9	7.9	5.5	5.0	Lysine	Hojgaard et al. (2019b)
19.3	8.3	5.8	4.8	Xylanase suppl.	Zhou et al. (2018)
20.0 to 20.6	8.9 to 9.4	5.6 to 6.1	4.8 to 4.9	Fiber & fat	Krogh et al. (2015)
20.3	9.2	6.1	4.6	+/− arginine	Krogh et al. (2016a)

CLA, conjugated linoleic acid.

Nutritional Impact on Colostrum Production

The impact of sow nutrition on colostrum production was mostly studied in the last 15 yr, and energy and lysine intakes seem to play important roles (see review by Theil et al., 2022). Colostrum yield of sows was recently shown to be maximal when sows consumed 3.0 kg of daily feed, which corresponds to a daily intake of 38.8 MJ ME and 23.9 g SID lysine (Feyera et al., 2021b). In a follow-up study, the impact of an intravenous glucose infusion was studied during nest building, and glucose infusion was initiated ½ to 1½ d when nest building behavior was evident and continued until 24 h after the onset of farrowing. The sows infused with glucose produced as much as 7.06 kg of colostrum (Nielsen et al., 2021), which is greater than reported in any previous study (Theil et al., 2022). However, it was not significantly higher than the colostrum yield for the control sows (6.71 kg) receiving a saline infusion. The experiment was carried out when ambient temperatures were very high (>30 °C), and sows performed very little nest building activity. It is, therefore, likely that glucose availability, and in turn glucose uptake by the mammary glands, was the limiting factor for colostrum yield.

Several studies have investigated the influence of high levels of fibers during the transition period on sow colostrum yield and piglet performance during the suckling period, and responses were highly variable depending on the fiber source. In a study by Theil et al. (2014a), sows were fed a standard gestation diet (17% dietary fiber) or one of three high-fiber diets (32% to 40% dietary fiber) originating from pectin residue, potato pulp, or sugar beet pulp. Sows were fed these diets from mating until day 108 of gestation, until they were transferred to the farrowing unit. Colostrum yield was lower in sows fed potato pulp when compared with sows from the other groups, whereas it was 20% and 24% greater in sows fed pectin residue compared with sows fed control or sugar beet pulp diets, respectively. Yet, these differences were not significant. In the same study, colostrum intake was significantly greater in piglets suckling sows fed sugar beet pulp or pectin residue (504 and 540 g/piglet, respectively) when compared with sows fed the control or potato pulp diets (414 and 339 g/piglet, respectively). Krogh et al. (2015) fed sows a control lactation diet with a fairly low content of dietary fiber (155 g/kg) or one of two high-fiber diets (230 g/kg) with the fiber originating from sugar beet pulp or alfalfa meal. Experimental diets were fed from day 105 of gestation to early lactation, but no improvements in colostrum yield were observed. On the other hand, colostral lactose (12 to 24 h) decreased in both high-fiber groups compared with the control group, and colostral dry matter at 24 h was greater in sows fed the sugar beet pulp-supplemented diet.

Feyera et al. (2021a) compared the impact of four fiber-rich diets containing 20.1%, 21.4%, 24.3%, and 19.7% dietary fibers with different origins (sugar beet pulp, soy hulls, palm kernel expellers or a mix of sugar beet pulp, oat hulls, and wood fiber, respectively) and fed from mating until farrowing. There was a numerically lower (13% to 16% lower) colostral yield and significantly lower colostral dry matter in sows fed palm kernel expellers than the remaining dietary groups. Quesnel et al. (2009) also reported a numerical increase in colostrum yield (3.4 vs. 3.0 kg) when sows were fed high fiber (11% crude fiber) when compared with 2.8% crude fiber in the control diet during gestation, although it should be emphasized that these authors used another prediction equation which is known to estimate lower colostrum intake of piglets and lower colostrum yield of sows (Theil et al., 2014a). High-fiber diets fed during the last 2 wk of gestation increased colostral fat content by 49% and increased the mammary uptake of acetate and butyrate in late gestation (days −7 and −3) and throughout the colostral period (Feyera et al., 2019). This increased uptake of ketogenic energy from acetate and butyrate most likely contributed to the increased fat concentration observed in colostrum secreted from sows fed the high-fiber diet. Loisel et al. (2013) observed greater colostrum intake (216 vs. 137 g/piglet, respectively) in low-birth-weight piglets (<900 g) suckling sows fed 23.4% dietary fiber compared with 13.3% dietary fiber from day 106 of gestation until farrowing, even though colostrum yield was not altered. These authors further reported a greater colostral fat content 24 h after farrowing in sows fed high crude fiber compared with low crude fiber (10.7% vs. 8.3%, respectively). Therefore, fiber-rich diets appear to have the potential to improve both yield and composition of colostrum, yet the response is variable depending on the fiber sources.

Liver Physiology

The liver has a double blood supply as it receives both arterial blood and venous blood through the portal vein. The latter comes from the gastrointestinal tract and is very rich in nutrients, especially during the postprandial phase. The liver has a high metabolic activity and even though it constitutes only 2% of the body weight (Kristensen and Wu, 2012), it accounts for 17%, 23%, and 16% of the total heat production of the sow on days −10, −3, and +3 relative to farrowing, respectively (Hu et al., 2020). The portal blood accounts for 73% to 76% of the total blood supply to the liver, whereas arterial blood accounts for the remaining 24% to 27%. In late gestation and early lactation (day −10 through day +3), the portal blood flow is approximately 228 L/h, and arterial supply is 80 L/h, whereby 308 L/h leaves the liver through the hepatic vein (Hu et al., 2020). The blood supply to the liver may account for as much as 40% of the cardiac output of blood (Kristensen and Wu, 2012). The liver has many roles and a crucial one is to control the energy metabolism and energy status of the animal. Thus, the liver has the ability to store glycogen which is released to maintain glucose homeostasis when needed. Most likely, the liver depot of glycogen is being depleted to some extent during farrowing because sows do not normally ingest feed while in labor, and this may explain why sows that coincidentally started their farrowing many hours after ingesting their last meal had problems with maintaining glucose homeostasis (Feyera et al., 2018). The liver is also responsible for oxidizing amino acids and converting them to metabolites that can enter the citric acid cycle and release urea that then will be excreted through the kidneys. This last statement is supported by the RQ value (ratio between CO₂ production and O₂ uptake) that ranges between 0.74 and 0.85 during the transition period (Hu et al., 2020). Amino acid oxidation can occur in different situations, for instance, if glycogenic energy (glucose and lactose) is scarcely available or if dietary amino acids are not well balanced. In a recent study, it was found that the liver oxidizes most amino acids during the first 1 to 2 h after consuming a meal and that even lysine (being the limiting amino acid) was oxidized (Hu et al., 2020). It is, however, difficult to understand why the liver would oxidize lysine under these conditions, when energy supply is high during the first few postprandial hours. The liver is also responsible for extracting propionate that originates from the fermentation of fibers in the hindgut and could then convert this propionate into lactate, which would be released to peripheral tissues such as muscles and the mammary glands (Theil et al., unpublished data). However, if glucose becomes strongly limited, such as during extended fasting, the liver metabolism is reversed. This was shown in fasted growing pigs exposed to an acute infusion of short-chain fatty acids into portal blood. Propionate was efficiently taken up by the liver, but, due to the low energy status, the liver also had a net uptake of lactate and converted propionate and lactate (which are both C-3 molecules) into glucose, which was then released to the peripheral circulation (Theil et al., 2016). The liver is indeed important for whole animal energy metabolism, but more research is needed on liver physiology to understand why this organ accounts for such a large proportion of the cardiac output and to understand how the physiology of the sow change around farrowing and in which ways nutrition of transition sows need to be elaborated.

Conclusion

The selection for large litters in the modern swine industry has led to increased metabolic demand on the sow throughout all stages of the reproductive cycles. The transition period from late gestation to early lactation is highly important because it can affect piglet survival and growth which are major determinants of sow productivity. Sows are undergoing marked physiological and metabolic changes during the transition period, and proper nutrition is needed to ensure optimal fetal and mammary growth, successful farrowing, and maximal colostrum production. The day of farrowing is critical in terms of meeting nutritional needs because energy and plasma glucose are limited resources, while sufficient energy is needed for the hyperprolific sows to farrow successfully and produce enough colostrum for all piglets in the litter. Indeed, the energy stores of sows may be inadequate if too many hours have elapsed from ingestion of the last meal before the onset of farrowing. This is the case because glucose is in great demand, being utilized for nest building activities, uterine contractions, and synthesis of colostrum. The glycogen depot in the liver may also become depleted, further exacerbating the situation. Sufficient energy may be provided via an adequate feed supply, more frequent daily meals, inclusion of high levels of fiber in the sow diet, or by supplementing sows with extra energy just prior to or during farrowing. While sufficient energy is needed for nest building and the farrowing process, proper mammary development and colostrum production seem to be more sensitive to a correct daily supply of amino acids.

Glossary

Abbreviations

AA

amino acid

Ig

immunoglobulin

IGF-1

insulin-like growth factor-1

ME

metabolizable energy

MJ

mega joule

RQ

respiratory quotient

SID

standardized ileal digestible

Conflict of interest statement

All the authors declare that there are no conflicts of interest.